1. Field of the Invention (Technical Field)
The present invention relates to rechargeable alkali metal and alkali metal ion power sources and methods of making cathode materials for such cells/batteries.
2. Background Art
Rechargeable alkali metal power sources, such as lithium cells/batteries, with either liquid or solid polymer electrolytes, exhibit high cell voltages and are capable of providing high energy and power densities over a wide operating temperature range. In recent years, considerable progress has been made in improving the performance characteristics of these power sources. These systems use alkali metal anodes and transition metal oxide or chalcogenide cathodes.
The commercial applications of power sources based on metallic lithium anodes is limited due to safety concerns, particularly under abuse conditions. The presence of metallic lithium in rechargeable cells/batteries also leads to premature cell failure due to dendritic growth. Recently, in order to address these concerns, many manufacturers, most notably Sony, have replaced metallic lithium with carbons capable of undergoing reversible lithium intercalation for use as the anode material. These power sources are commonly known as lithium ion cells and/or batteries, and they generally use solid transition metal oxides and chalcogenides as the cathode materials.
Metal oxides and chalcogenides, such as cobalt oxide (LiCoO.sub.2), manganese oxide (LiMn.sub.2 O.sub.4), nickel oxide (LiNiO.sub.2), vanadium oxide (V.sub.6 O.sub.13), and cobalt disulfide (CoS.sub.2), have been under development as cathode materials in both lithium and lithium ion cells and batteries. These materials are capable of undergoing reversible intercalation of lithium ion upon electrochemical oxidation and reduction, i.e., their charge and discharge. These oxides, when used in conjunction with carbon materials which can be reversibly intercalated with lithium, makes possible the development of a true "rocking chair" cell, in which lithium ion reversibly intercalates/deintercalates in both the anode and cathode active materials.
Rocking chair batteries exhibit improved ambient temperature performance compared to most other common rechargeable battery systems, such as Ni/Cd, Pb/acid, and alkaline batteries. However, when these systems are stored and/or operated at higher temperatures their performance deteriorates. One of the reasons for this degradation of performance appears to be the instability of the transition metal oxide/chalcogenide cathode materials. Evaluation of published data show that metal oxides in general deteriorate on electrochemical cycling. The extent of irreversible loss of capacity per unit weight of cathode material appears to depend on the rechargeable cycle life, operating temperature, current (i.e., the rate), and the electrolyte used. It is believed that this capacity loss is caused by the structural changes taking place in the crystal structure of the metal oxides during repeated charge/discharge cycling. The newly formed oxide(s) appears to be electrochemically inactive. Consequently, the successful development of these lithium ion power sources for both consumer and electric vehicle applications depends to a large extent on the synthesis of stable metal oxides and chalcogenide materials.
In addition, the performance characteristics of the cell/battery, e.g., cycle life, rate capability, energy density, etc., are dictated, for the most part, by the intended application for the cell/battery. For example, a pacemaker is a low rate device requiring very high reliability, while a cell/battery for automotive applications is a higher rate device requiring somewhat less reliability. These performance characteristics are influenced by the morphological characteristics, such as particle size and surface area, of the active materials. In general, the desired particle size and surface area of the cathode materials is obtained after preparation of the material using mechanical means. Hence, the development of lithium and lithium ion power sources for various applications depends, to some extent, on the ability to prepare materials having the appropriate morphology.
Furthermore, in lithium cells/batteries, and/or in lithium ion cells where there is the possibility of depositing metallic lithium, safety of the system is of concern primarily due to the high reactivity of metallic lithium. In the event of cell shorting, the heat generated as a result of the rapid oxidation/reduction reaction exacerbates the condition, leading to a possible runaway reaction and violent cell decomposition. These safety concerns must be adequately addressed.
The present application discloses a method of making such stable materials of the desired morphological characteristics, as well as materials that incorporate improved overall safety features of a cell/battery.